Referring to FIGS. 1 to 4, there is shown a conventional disc cartridge, wherein a top half 1a and a bottom half 1b are assembled together to form a disc cartridge 1 and a magnetic floppy disc 2 is rotatably enclosed in a chamber formed between the top half 1a and the bottom half 1b. A shutter 3 having a pair of opposed plates 3a and 3b connected by a front plate 3c as shown in FIG. 3, is movably fitted to the recessed portion 4 of the disc cartridge 1 for opening and closing a head insertion hole 7. In the process of mounting the shutter 3 to the disc cartridge 1 from the front, the cartridge case is fitted in the gap between both plates 3a and 3b of the shutter 3 and the shutter 3 is moved backward, a projection 5 formed on the shutter 3 projected inwardly, deforms the wall of the recessed portion 4, thus the projection 5 is slidably engaged in a guide slot 6. However, the resistance to the deformation of the recessed portion is high since the wall of the recessed portion 4 is in the form of a beam supported at both side edges by the side walls of the disc cartridge, having a flat plate member; therefore, the recessed portion 4 is not easily deformed by the projection 5 in the process of mounting the shutter 3, whereby the force for inserting the shutter 3 changes from one disc cartridge to another, and there tends to occur incorrect mounting of the shutter to the disc cartridge.